Two main systems are used in existing automated bean to cup coffee dispensing machines for the production of hot water and steam. In the first type the hot water and the steam are created in a single boiler. In the second type the beverage machines comprise two boilers which have different functions. One boiler (water boiler) is used to create the hot water required to brew the coffee (or to dispense hot water into the cup) the other boiler (steam boiler) is used to create the steam required to heat the milk and produce milk foam.
In the two boiler systems, coffee machines tend to operate with a single filtered water supply, with the filtered water split between the steam and water boilers, as required. There are four main types of cartridge filters that can be used in these machines:                activated carbon only (used for soft or pre-treated water to provide filtration of particulates, organics and, for example, chlorides);        activated carbon plus mineral addition (used for soft or pre-treated water only where the user also wishes to increase the mineral content for reasons of taste profile in the drink or conductivity);        activated carbon plus hydrogen anion exchange (for harder water where the user also wishes to reduce the build-up of limescale through the removal of carbonate ions); and        activated carbon plus sodium cation exchange (for hard or gypsum waters where the user wishes to reduce the build-up of limescale or gypsum through replacement of calcium and magnesium ions with sodium ions that do not precipitate out as limescale/gypsum in the tanks).        
The per litre cost of filtration is strongly dependent upon the type of filtration used. Carbon only filters have a low cost per litre (currently around 0.16 pence per litre), hydrogen filters have a medium cost per litre (currently around 1.5p per litre), sodium filters have a high cost per litre (currently around 2.5 p per litre). Carbon filters with mineral addition also have high cost per litre. A typical bean to cup coffee machine will use about 20 litres of water per day (based on a 50 cup per day usage rate).
The chemical composition of the water entering the coffee making modules of a beverage dispensing machine is determined by the composition of the incoming water to the filter and the effect of the filter in modifying the composition before it enters the coffee making modules. However the preferred water composition required by the different water systems within the coffee machine varies. Therefore the use of a single filter water system can led to a compromise between the requirements of the water used for making the coffee and the water used to make steam for the milk.
The water used for the brewing of coffee requires relatively high mineral content in order to develop the correct tastes and aromas of the coffee. This water is held in liquid form throughout the system and is generally in the temperature range 92° C. to 96° C. As the water used to produce coffee flows in and out of the coffee boiler in the same physical state the concentration of minerals within the boiler is largely in a steady state, controlled by the incoming water and the operation of the filter.
The water used to produce steam for the production of hot milk and hot milk foam requires minimal mineral content, especially regarding the presence of Ca2+ and Mg2+ cations, and CO32− and SO42− anions, to prevent the formation of calcium and magnesium carbonates (limescale) and calcium sulphate (gypsum) within the steam tank. The saturation levels of harmful minerals in the tank decreases with increasing temperature, and therefore as the steam boiler runs at around 135° C. the propensity for minerals to deposit is higher in the steam boiler than in the coffee boiler.
In current water systems for beverage dispensing machines the mineral content in the steam boiler increases with cups dispensed between boiler blow-down cycles (when the water in the boiler is changed, normally at a daily clean). This is because the infeed water in the liquid phase contains minerals, whilst the outgoing fluid in the gaseous phase contains little or no minerals. This results in an increase in mineral content over time in the steam boiler as steam is used to heat milk for coffees and automatic machine rinses are performed. Once the mineral content goes above a threshold level in the steam boiler limescale (and gypsum) will form, which inhibits the performance of the steam system causing the system to require a maintenance operation to rectify. Scale build-up in the steam tank can result in failure of the steam temperature probe, failure of the water level probe, reduced effectiveness of the heater element, reduced volume of the tank resulting in a reduced head of steam, reduced flow rate into the tank through the water fill tube, increased energy costs and/or cold or under-filled drinks.
Once the limescale or gypsum has started to form in the steam tank it is not possible to remove it in normal running. If a hydrogen filter has been used replacement of the filter will reduce pH leading to break up of the scale into small particles which may lodge in the steam control valves downstream. Remedial action is normally carried out to either replace the affected parts at a high cost, or to run a descaling process in situ involving the use of aqueous acids to dissolve the limescale in the tank. Removed parts may be descaled chemically away from the installation through a refurbishment route. However repeated descaling may eventually lead to the boiler parts being scrapped due to galvanic corrosion between the dissimilar metals used to fabricate the different parts of the tank assembly.
The current filtration systems used in beverage dispensing machines, particularly in a hard or moderately hard water area can be a compromise between coffee quality, machine reliability and cost. A single sodium filter will give poorer coffee quality but higher machine reliability at a higher filter cost, a single hydrogen filter will give better coffee quality and lower machine reliability at a lower filter cost.
The invention provides an alternative filtration system suitable for use in two boiler beverage dispensing machines.